Present practice in the aircraft industry is to build airplanes out of components that are joined together by rivets. The heads of the rivets at the surfaces of the airplane experiencing high velocity airflow are usually made substantially flush with the surfaces. This results in substantially less aerodynamic drag than would otherwise be generated if raised-head rivets were used.
The surface components usually are provided with dimpled, countersunk or otherwise recessed holes that receive the rivet heads to be installed in a flush condition. The most prevalent procedure for flush riveting is to install rivets having oversized heads. The extra material of the oversized head is then shaved off until the head is nearly flush with the surrounding surface. This "nearly" rather than absolutely flush condition, though undesirable, is dictated by the need of a production tolerance to assure that damage to the surface adjacent to the rivet head is avoided since structural damage occurs when rivet heads are inadvertently shaved below the surface. The protruding edge of the shaved rivet head often presents a sharp edge to the high velocity airflow which is a source of drag that can be unexpectedly large especially when the sharp edge is raised by yielding of the riveted joint. Also it is difficult to assure the complete expansion of the head and shank of a rivet designed to be shaved. Once the rivet is installed, no satisfactory non-destructive test is available to find the poorly installed rivets. Thus, during the design process, designers must take into consideration that some walls of the rivet holes are not expanded to optimum size by the rivet which means components must be overdesigned.
The installation difficulty of the prior art rivets is usually caused by the overlapping of rivet head periphery which is forced by the rivet hammer against the surface adjacent to the edge of the countersink, thereby supporting the hammer and preventing the desired flow of rivet head material into the countersink and hole. As has been proved by comparative tests which show the superiority of the present invention, the lack of rivet head material flow on the shaved flush rivet results in much lower fatigue strengths of the riveted joints. Another problem with the shaved flush rivet is the shaving procedure removes the center indent or other indication normally provided to identify the material from which the rivet is made. Also, when a shaved head rivet is inadvertently installed into a countersink which is too shallow, the shaving operation destroys the inspectability of the error and removes too much of the rivet head. This can cause the head to fail by "cupping" in service.
Another cause for concern with the prior art rivets is that shaving rivets made from monel and other relatively hard materials required in modern titanium structures, costs five times more than shaving aluminum rivets. At the present, the cost of shaving aluminum rivets on large commercial aircraft is about $20,000 per aircraft.